1. Field of the Invention
The present invention relates, in general, to head drums for use in magnetic tape units and, more particularly, to a low-frictional-drag head drum configuration suited to use in helical-scan-type magnetic tape units.
2. Description Relative to the Prior Art
The movement of magnetic tape over a head drum is a critical aspect of the recording and the reading of magnetic records. As might be expected, there has been extensive development in the area of head drum design and various approaches have been taken to achieve desired head drum characteristics.
U.S. Pat. No. 3,428,524, for example, shows a helical-scan-type head drum wherein air ports are provided for forcing air beneath the magnetic tape so as to form an air bearing. By so forcing air under the tape, resistance to longitudinal tape movement over the stationary drum is greatly reduced. Such an arrangement, however, is quite complex and expensive.
Another type of drum utilizes a cylindrical drum unit which rotates at high speed (the speed of the magnetic heads). Because the drum rotates at high speed, an air film is created between the drum and the tape. With such a design, however, accurate guiding of the tape past the heads becomes difficult because the tape tends to float on the drum.
U.S. Pat. No. 3,510,604 describes a configuration wherein a rotating drum is specially tapered so as to cause air to be forced between the tape and the drum surface to produce an air bearing.
A split drum configuration is described in U.S. Pat. No. 3,436,491, which configuration utilizes an inwardly tapered upper drum section having a diameter at its bottom face that is less than the diameter of an adjacent stationary lower drum section. The taper is said to reduce frictional drag by reducing the buildup of oxides which are rubbed from the tape by the drum. This arrangement, as described, is intended for a helical-scan apparatus having a tape wrap angle approaching 360.degree..
U.S. Pat. No. 3,840,895 describes a split drum configuration having a rotating lower section which carries a set of heads and a coaxial, increased diameter upper section which is journaled so as to be rotated by the magnetic tape passing thereover. Because guide ridges are provided on the drum sections along the edges of the tape path, which ridges rotate with their respective sections, a circular, rather than helical, tape path must be utilized. Moreover, it is noted that the torque required to rotate the upper section acts as a drag on longitudinal tape movement with this arrangement.
As will be appreciated, subtle changes in head drum design can result in significant differences in performance characteristics. Consequently, various drum configurations have been developed and those described above represent only a sampling. Each configuration has advantages and disadvantages which are related to a degree to the overall tape scanning system in which the configuration cooperates.
For use in a cartridge video apparatus, a simple helical-scan head drum which requires no special air supply or elaborate tape guides would be desirable. An overall tape scanning system which operates at low tension to reduce head wear would also be desirable, but the tension should be uniform along the tape wrap path to assure uniform head response. It would further be desirable, particularly in a cartridge-type tape scanning system, to provide for protection of the heads and head drum in the event the tape supply jams.